'A fingerprint is like a password that you write down wherever you go' Photograph: Toshiyuki Aizawa /REUTERS

It takes Molly and Piotr three days to get ready. He spends the second day at her kitchen table, looking through her video footage and making notes in a neat, careful hand.

Molly had told her dad that he was building her a website; Cyril Root had just peered at Piotr over the tops of his half-moon reading glasses, offered him a cup of tea, and then shuffled off down to his workshop to tease cogs and springs into place with a pair of tiny tweezers.

If Molly had been worried about the fingerprint scanner, Piotr is simply amused. "It is security theatre. People see spy movies and think it's high-tech, it's a good way to lock doors. It's not. In my humble opinion, at least."

"Security theatre?"

"You know, something that looks like it is making you safer when it's really just for show. Like taking away your nail clippers when you fly on a plane. All you need to open this lock is the right finger."

He laughs. "Of course not. The point is not to be detected. All we need is the print, and that's easy to get. It's what makes this a not so good system - a fingerprint is like a password that you write down wherever you go, after all."

"And we've got Gavin's?"

Piotr reaches down and takes Molly's laptop out of her bag, flipping it over. "We should have some good ones on the battery, from what you tell me. He took it out, yes?"

Molly nods. Piotr carefully unlatches the dead battery and seals it inside a clean plastic bag. "OK. Let us see if your eminent father has a few supplies . . . "

They spend the next hour lifting the prints. Piotr builds a sealed plastic box in the back garden big enough to contain the laptop battery and a small dish of liquid heated with a chafing dish. White fumes fill the box; Piotr allows the to heat die down and takes the lid off, letting them disperse. There's a noxious smell that makes Molly glad they're standing well back. When it's safe, they take the battery back inside and dust it with powdered chalk. A dozen or so prints show up immediately, white against the dark grey of the casing.

"The chalk sticks to the print better now, you see? I think this is Gavin - these here look like yours, they're smaller." Piotr takes Molly's camera, focuses carefully, and takes pictures.

They go upstairs, fire up Molly's PC, and turn the pictures into negatives, black on white. She prints them on to sheets of clear plastic they've bought for this purpose, blowing them up so that they can fill in any broken lines, and then re-photographing them. At the end of it, they have a sheet of sharp black prints, exactly the same size as before.

"They're looking good," Molly says. "What do you think - do we need to etch them?"

Piotr is inspecting the sheet with a jeweller's loupe screwed into his left eye; when he smiles, it gives him a sinister look, like a Bond villain. "This might work on its own - it depends how bad their bad security is! But I think we must go one step further, to be safe."

"OK. You take care of that; I'll go and check on the video." She means the camera they'd set up the day before, overlooking the Harris, Renfield and Church parking lot. From that position, it has a clear view of all the firm's windows; Molly and Piotr are watching, analysing, looking for patterns.

She takes her bike on the train out to Ealing and cycles down the road that the parking lot backs on to. The camera's still in place, tucked into an old orange juice carton left on top of the seven-foot wall that separates the two. It's mid-afternoon, and nobody should be leaving the solicitors' office for a while yet; Molly checks both ways, sees nobody on the street, and manages to reach it down.

She swaps out the memory card and the battery for fresh ones, stands up on her bike pedals, and replaces it as carefully as she can. Piotr, you should be doing this - you're the tall one, she thinks. Mind you, about now Piotr is messing about with dangerous chemicals, and on balance that's the way she prefers it.

Piotr needs to make a rubber cast of Gavin's fingerprint, to wear over his own. He's using a trick that he learned from the kind of printed circuits that they use in electronics. The clear plastic with the print on it goes over an unused circuit board, and is exposed to ultraviolet light, which hardens the board's coating - except for the parts under the print itself, which are in shadow. Then there's a complicated series of caustic baths, to remove everything but the raised mirror image of the print.

By the time Molly gets back to north London, he's standing in the kitchen casting it in a clear gel; he's pouring it into a baking ring that Molly uses to make muffins. At his feet, Stanley is staring up at him patiently, hoping it is some new kind of food.

"How's it looking?" she asks.

"Hello – not bad. I think this will work, if it sets well. Did you get the video?"

"Yep." Molly fishes the card out of her pocket while Piotr finishes the pour. "Shall we take a look?"

***

On the third night, just before ten, Piotr drives them out to Ealing in a sleek, midnight blue Audi. It smells brand new inside; the security business is working out well for him, she thinks. They're dressed in dark colours. "Not too much black," Piotr had said. "Two people dressed all in black is burglary - people remember. We are coming back from a club, perhaps." He is still in his black leather jacket, but she is wearing a dark green bomber jacket that her aunt had sent her for Christmas once. She feels odd, out of her usual monochrome.

They park up beside the wall with the camera, retrieve it, and check the footage. The pattern Molly has noticed, reviewing the last few days of video, seems to be holding. There's still one light on in a third-floor office - one lawyer burning the midnight oil, as he has been every night so far. He leaves about ten thirty. There are still three cars in the parking lot, but from what they can tell two of them never move.

They've got about ten minutes. Molly helps Piotr scramble over the wall and chucks a reel of line over to him. Then she waits.

He takes less than a minute, but every moment seems like an hour. Her heart is hammering in her chest. She looks left, looks right - nothing. In the distance a woman walks her dog towards the Broadway, but she doesn't look in Molly's direction.

She hears a whisper from behind her. "OK." She grabs the old orange crate out of the back of Piotr's car, climbs up on it, and boosts herself up to the top of the wall, where Piotr grabs her and helps her over; she lands in a drift of autumn leaves with a crunch that makes her wince.

Piotr's smile is very bright in the moonlight; his eyes are lit up. This is what he really enjoys, Molly thinks. He's having the time of his life.

They crouch in the shadow of the rearmost car in the lot. Molly checks her watch and almost on cue the light in the office window goes out.

"Get ready," she whispers.

Two minutes later, the fire door in the back of the building opens and the hard-working solicitor emerges. It's the exit that comes from the back stairs - every night, he comes out this way, gets in his car, and drives home.

Clockwork, she thinks. The fire door starts to swing shut behind him as he trudges off, jingling his keys.

Piotr pulls on the reel of high-tension fishing line. It runs under the car in front of them and diagonally across the lot. The other end is pinned securely to the door, and as it goes taut she can see the effort it's taking to hold it from closing all the way. He lets it go just far enough that there's an audible click as it touches the lock plate, but not far enough that it latches, and hangs on.

They watch as the solicitor starts up his car and noses out of the lot through the gate to Uxbridge Road. As soon as he is out of sight, Piotr exhales with relief, and starts to pull the line in.

Seconds later, they're in the building; the fire exit stairwell. The door directly ahead leads to the corridor with the lifts where Molly had come in the other day. Piotr opens it a crack, and puts a dentist's mirror through to check the security desk.

"Wait," he whispers. "Almost there."

Two minutes later he closes the door and turns to face her. "He's off on his rounds. We go now, yes?"

They go out of the door and into the hall. Molly sees the lift indicator click up to 1; the guard on the night desk has his routines as well. They've seen him walking the upstairs offices on a regular schedule. Molly estimates they have ten minutes.

As they approach the IT office, Piotr reaches into his jacket and takes out a plastic tube containing the cast. "Let's see if this works," he mutters.

He wraps the scrap of rubber around his index finger, presses it to the scanner. There's a beep, and a click…

Molly tries the handle and the door swings open.

The next instalment of Root will be available on MondayIf you can't wait till then, join the discussion on our Facebook page

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